Multiplexer, high-frequency front end circuit, and communication device

ABSTRACT

A multiplexer ( 1 ) includes a plurality of filters connected to a common terminal ( 110 ). The multiplexer ( 1 ) includes: a low-frequency filter ( 11 L) that is formed of at least one surface acoustic wave resonator arranged between the common terminal ( 110 ) and the input/output terminal ( 120 ) and has a first pass band; a high-frequency filter ( 12 H) that is connected between the common terminal ( 110 ) and the input/output terminal ( 130 ) and has a second pass band located at a higher frequency than the first pass band; and a capacitor (C B1 ) that is serially arranged in a connection path between the common terminal ( 110 ) and the low-frequency filter ( 11 L). The Q value of the capacitor (C B1 ) in the second pass band is higher than the Q value in the second pass band of a capacitance obtained by treating the at least one surface acoustic wave resonator of the low-frequency filter ( 11 L) as a capacitance.

This is a division of U.S. patent application Ser. No. 16/217,254 filedon Dec. 12, 2018, which is a continuation of International ApplicationNo. PCT/JP2017/019261 filed on May 23, 2017 which claims priority fromJapanese Patent Application No. 2016-118399 filed on Jun. 14, 2016. Thecontents of these applications are incorporated herein by reference intheir entireties.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The present disclosure relates to a multiplexer that includes surfaceacoustic wave filters, to a high-frequency front end circuit, and to acommunication device.

Description of the Related Art

For recent cellular phones, there has been a demand for a plurality offrequency bands and a plurality of wireless systems, so-called multiplebands and multiple modes, to be supported by a single cellular phoneterminal. In order to realize this demand, a multiplexer that splits ahigh-frequency signal having a plurality of wireless carrier frequenciesis arranged immediately below a single antenna. Surface acoustic wavefilters having low loss inside the pass band thereof and a steepbandpass characteristic around the periphery of the pass band are usedas the plurality of band pass filters constituting such a multiplexer.

Patent Document 1 discloses a surface acoustic wave filter device havinga configuration in which a plurality of surface acoustic wave filtersare connected to each other.

FIG. 10 is a circuit configuration diagram of a surface acoustic wavefilter device 501 disclosed in Patent Document 1. Specifically, atransmission filter 520 and a reception filter 513, which are formed ofsurface acoustic wave resonators, are commonly connected to an antennaterminal 510, and an impedance-matching shunt inductor L is connected tothe antenna terminal 510. The transmission filter 520 is, for example, aUMTS Band 3 transmission filter (transmission band: 1710-1785 MHz) andthe reception filter 513 is, for example, a UMTS Band 3 reception filter(reception band: 1805-1880 MHz).

-   Patent Document 1: Japanese Unexamined Patent Application    Publication No. 2012-175315

BRIEF SUMMARY OF THE DISCLOSURE

However, in the case where the transmission filter 520 and the receptionfilter 513 are commonly connected to the antenna terminal 510 as in thesurface acoustic wave filter device 501 disclosed in Patent Document 1and in the case where a plurality of transmission filters and aplurality of reception filters are commonly connected to an antennaterminal in order to support multiple bands, the filter characteristicof one filter is greatly affected by the filter characteristic ofanother filter. For example, in the case where the return loss seen fromthe antenna terminal side of the other filter is increased in the passband of the one filter, the insertion loss in the pass band of the onefilter is increased by the reflection characteristic of the otherfilter. If a demultiplexing/multiplexing device, a phase adjustingcircuit, or the like is arranged in a stage prior to the filters as acountermeasure, the multiplexer is undesirably increased in size andcost.

Accordingly, the present disclosure was made in order to solve theabove-described problem, and it is an object thereof to provide amultiplexer, a high-frequency front end circuit, and a communicationdevice in which the propagation loss of a high-frequency signal isreduced without arranging a demultiplexing/multiplexing device, aphase-adjusting circuit, or the like in a stage prior to the filters,and that are compact and low cost.

In order to achieve this object, a multiplexer according to an aspect ofthe present disclosure has a common terminal, a first input/outputterminal, and a second input/output terminal, and includes a pluralityof filters connected to the common terminal. The multiplexer includes: afirst filter that is formed of at least one surface acoustic waveresonator arranged between the common terminal and the firstinput/output terminal, and has a first pass band; a second filter thatis connected between the common terminal and the second input/outputterminal and has a second pass band located at a higher frequency thanthe first pass band; and a capacitor that is serially arranged on aconnection path between the common terminal and the first filter. A Qvalue of the capacitor in the second pass band is higher than a Q valuein the second pass band of a capacitance obtained when treating the atleast one surface acoustic wave resonator as a capacitance.

It is known that the loss due to bulk waves is generated on thehigh-frequency side of an anti-resonant point in a SAW resonator thatutilizes leaky waves using a lithium tantalate (LiTaO₃) substrate orLove waves using a lithium niobate (LiNbO₃) substrate. In other words,in a high-frequency filter formed of such a surface acoustic waveresonator, the loss due to bulk waves is generated in an attenuationband on the high-frequency side of the center frequency. This loss dueto bulk waves substantially does not affect the attenuation of thehigh-frequency filter, but the reflection coefficient (|Γ|) of thehigh-frequency filter is reduced. On the other hand, a high-frequencyfilter formed of a surface acoustic wave resonator has a capacitivecharacteristic and functions as a capacitor, and therefore functions asa capacitor having a low Q value in the frequency band in which the bulkwaves are generated. Therefore, in the case of a multiplexer having aplurality of filters connected to a common terminal, the bulk wave lossof a first filter having a low pass band (first pass band) causes theinsertion loss inside the pass band of a second filter having a passband (second pass band) that contains the frequency at which the bulkwave loss is generated to increase.

In contrast, according to the above-described configuration, a capacitorhaving a Q value that is higher than a capacitance Q value, in thesecond pass band, of the at least one surface acoustic wave resonatorconstituting the first filter is inserted between the first filter andthe common terminal. Therefore, the capacitance Q value in the secondpass band can be increased on the common terminal side of the firstfilter. As a result, the reflection coefficient (|Γ|) of the firstfilter in the second pass band can be increased (return loss can bereduced), and therefore the insertion loss in the second pass band ofthe second filter, which is connected to the common terminal along withthe first filter, can be improved. Therefore, the propagation loss of ahigh-frequency signal can be reduced without arranging ademultiplexing/multiplexing device, a phase adjusting circuit, or thelike in a stage prior to the filters, and the reductions in size andcost can be realized.

In addition, a multiplexer according to an aspect of the presentdisclosure has a common terminal, a first input/output terminal, asecond input/output terminal, and includes a plurality of filtersconnected to the common terminal. The multiplexer includes: a firstfilter that is formed of at least one surface acoustic wave resonatorarranged between the common terminal and the first input/outputterminal, and has a first pass band; a second filter that is connectedbetween the common terminal and the second input/output terminal, andhas a second pass band located at a higher frequency than the first passband; and a capacitor that is serially arranged on a connection pathbetween the common terminal and the first filter and is for compensatingfor bulk wave radiation loss of the at least one surface acoustic waveresonator in the second pass band.

According to this configuration, a capacitor for compensating for thebulk wave radiation, in the second pass band, of the at least onesurface acoustic wave resonator constituting the first filter isinserted between the first filter and the common terminal. As a result,the reflection coefficient (|Γ|) of the first filter in the second passband can be increased (return loss can be reduced), and therefore theinsertion loss in the second pass band of the second filter can beimproved. Therefore, the propagation loss of a high-frequency signal canbe reduced without arranging a demultiplexing/multiplexing device, aphase adjusting circuit, or the like in a stage prior to the filters,and the reductions in size and cost can be realized.

In addition, the first filter may have a substrate having apiezoelectric property in at least part thereof and on which aninterdigital transducer (IDT) electrode constituting the at least onesurface acoustic wave resonator is formed, the capacitor may be formedon the substrate using comb-shaped electrodes that face each other, anda pitch of a plurality of electrode fingers constituting the comb-shapedelectrodes of the capacitor may be smaller than a pitch of a pluralityof electrode fingers constituting the IDT electrode.

According to this configuration, the frequency of the bulk waveradiation generated by the capacitor is shifted so as to be furthertoward the high-frequency side than the frequency of the bulk waveradiation generated by the IDT electrode, and therefore the Q value ofthe capacitor in the second pass band can be set so as to be higher thanthe capacitance Q value of the at least one surface acoustic waveresonator in the second pass band. Therefore, the capacitance Q value inthe second pass band can be increased on the common terminal side of thefirst filter. As a result, the reflection coefficient (|Γ|) of the firstfilter in the second pass band can be increased (return loss can bereduced), and therefore the insertion loss in the second pass band ofthe second filter can be improved.

In addition, the capacitor may be formed in a plurality of layers insidea multilayer substrate on which the first filter and the second filterare mounted and in which a wiring line that connects the first filterand the second filter to each other is formed.

According to this configuration, in contrast to the case where the IDTelectrode of the at least one surface acoustic wave resonator is formedon one surface of the substrate, the capacitor has a multilayerstructure, and therefore the Q value of the capacitor in the second passband can be set so as to be higher than the capacitance Q value of theat least one surface acoustic wave resonator in the second pass band.Therefore, the capacitance Q value in the second pass band can beincreased on the common terminal side of the first filter. As a result,the reflection coefficient (|Γ|) of the first filter in the second passband can be increased (return loss can be reduced), and therefore theinsertion loss in the second pass band of the second filter can beimproved.

In addition, in the at least one surface acoustic wave resonator of thefirst filter, leaky waves that propagate along a piezoelectric substratecomposed of LiTaO₃ may be utilized as surface acoustic waves.

In a surface acoustic wave resonator in which leaky waves are utilizedusing a piezoelectric substrate composed of LiTaO₃, the loss due to bulkwaves is generated on the high-frequency side of the anti-resonantpoint.

According to this configuration, the reflection coefficient (|Γ|) of thefirst filter in the second pass band can be increased (return loss canbe reduced), and therefore the insertion loss in the second pass band ofthe second filter can be improved. Therefore, the propagation loss of ahigh-frequency signal can be reduced without arranging ademultiplexing/multiplexing device, a phase adjusting circuit, or thelike in a stage prior to the filters, and the reductions in size andcost can be realized.

In addition, in the at least one surface acoustic wave resonator of thefirst filter, Love waves that propagate along a piezoelectric substratecomposed of LiNbO₃ may be utilized as surface acoustic waves.

In a surface acoustic wave resonator in which Love waves are utilizedusing a piezoelectric substrate composed of LiNbO₃, the loss due to bulkwaves is generated on the high-frequency side of the anti-resonantpoint.

According to this configuration, the reflection coefficient (|Γ|) of thefirst filter in the second pass band can be increased (return loss canbe reduced), and therefore the insertion loss in the second pass band ofthe second filter can be improved. Therefore, the propagation loss of ahigh-frequency signal can be reduced without arranging ademultiplexing/multiplexing device, a phase adjusting circuit, or thelike in a stage prior to the filters, and the reductions in size andcost can be realized.

The multiplexer may further include: a third input/output terminal; anda third filter that is formed of at least one surface acoustic waveresonator arranged between the common terminal and the thirdinput/output terminal, and has a third pass band located at a lowerfrequency than the second pass band. In the at least one surfaceacoustic wave resonator of the third filter, Rayleigh waves thatpropagate along a piezoelectric substrate composed of LiNbO₃ may beutilized as surface acoustic waves, and a capacitor does not have to beserially arranged on a connection path between the common terminal andthe third filter.

In a surface acoustic wave resonator utilizing Rayleigh waves using apiezoelectric substrate composed of LiNbO₃, the frequency at which bulkwave radiation is generated on the high-frequency side of theanti-resonant point lies in a frequency band located at a frequency atleast twice the frequency of the anti-resonant point and is sufficientlyhigher than the pass band of a filter used in a cellular phonemultiplexer, and there is substantially no effect from the bulk waveradiation on other filters.

According to this configuration, the reflection coefficient (|Γ|) of thethird filter in the second pass band is not reduced by the bulk waveradiation. Therefore, the propagation loss of a high-frequency signalcan be reduced without arranging a capacitor in a stage prior to thethird filter and the reductions in size and cost can be realized.

The multiplexer may further include an inductor that is connectedbetween the common terminal and the ground terminal.

Thus, impedance matching can be secured between the antenna element andeach filter.

In addition, a high-frequency front end circuit according to an aspectof the present disclosure includes: any one of the multiplexersdescribed above; and an amplification circuit that is connected to themultiplexer.

According to this configuration, a high-frequency front end circuit canbe provided in which the propagation loss of a high-frequency signal canbe reduced without arranging a demultiplexing/multiplexing device, aphase adjusting circuit, or the like in a stage prior to the filters,and in which the reductions in size and cost can be realized.

In addition, a communication device according to an aspect of thepresent disclosure includes: an RF signal processing circuit thatprocesses a high-frequency signal transmitted or received by an antennaelement; and the above-described high-frequency front end circuit thattransmits the high-frequency signal between the antenna element and theRF signal processing circuit.

According to this configuration, the propagation loss of ahigh-frequency signal can be reduced without arranging ademultiplexing/multiplexing device, a phase adjusting circuit, or thelike in a stage prior to the filters, and the reductions in size andcost can be realized.

With a multiplexer, a high-frequency front end circuit, or acommunication device according to the present disclosure, thepropagation loss of a high-frequency signal can be reduced withoutarranging a demultiplexing/multiplexing device, a phase adjustingcircuit, or the like in a stage prior to the filters, and the reductionsin size and cost can be realized.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a circuit configuration diagram of a multiplexer according toembodiment 1.

FIG. 1B is a circuit configuration diagram of a multiplexer according tomodification 1 of embodiment 1.

FIG. 2 is a circuit configuration diagram of a multiplexer according toa comparative example.

FIG. 3A is a graph illustrating a wide bandpass characteristic of alow-frequency filter.

FIG. 3B illustrates a graph depicting a wide reflection characteristicof the low-frequency filter.

FIG. 3C is a polar chart illustrating a reflection characteristic of thelow-frequency filter.

FIG. 4A is a graph illustrating a wide bandpass characteristic of ahigh-frequency filter according to embodiment 1.

FIG. 4B is a graph illustrating a wide bandpass characteristic of ahigh-frequency filter according to the comparative example.

FIG. 5A is a diagram illustrating the relationship between the Q valueof an equivalent capacitance component in a stage prior to ahigh-frequency filter, the reflection coefficient of a low-frequencyfilter, and the insertion loss of the high-frequency filter.

FIG. 5B is a graph illustrating the relationship between the Q value ofthe equivalent capacitance component in the stage prior to thehigh-frequency filter and the insertion loss of the high-frequencyfilter.

FIG. 5C is a graph illustrating the relationship between the reflectioncoefficient of the low-frequency filter and the insertion loss of thehigh-frequency filter.

FIG. 5D is a graph illustrating the relationship between the Q value ofthe equivalent capacitance component in the stage prior to thehigh-frequency filter and the reflection coefficient of thelow-frequency filter.

FIG. 6A is a plan view illustrating a first example of the electrodelayout of a low-frequency filter and a capacitor according to embodiment1.

FIG. 6B is an example of an external perspective view of the multiplexeraccording to embodiment 1.

FIG. 6C is a sectional view illustrating a second example of theelectrode layout of a low-frequency filter and a capacitor according toembodiment 1.

FIG. 7A is a circuit configuration diagram of a multiplexer according tomodification 2 of embodiment 1.

FIG. 7B is a circuit configuration diagram of a multiplexer according tomodification 3 of embodiment 1.

FIG. 8A is a circuit block diagram of a multiplexer according toembodiment 2.

FIG. 8B is a circuit configuration diagram of the multiplexer accordingto embodiment 2.

FIG. 8C is a circuit configuration diagram of a multiplexer according tomodification 1 of embodiment 2.

FIG. 8D is a circuit configuration diagram of a multiplexer according tomodification 2 of embodiment 2.

FIG. 8E is a circuit block diagram of a multiplexer according tomodification 3 of embodiment 2.

FIG. 8F is a circuit configuration diagram of a multiplexer according tomodification 3 of embodiment 2.

FIG. 9 is a circuit configuration diagram of a high-frequency front endcircuit and a communication device according to embodiment 3.

FIG. 10 is a circuit configuration diagram of a surface acoustic wavefilter device disclosed in Patent Document 1.

DETAILED DESCRIPTION OF THE DISCLOSURE

Hereafter, embodiments of the present disclosure will be described indetail using the drawings. The embodiments described hereafter eachillustrate a comprehensive or specific example of the presentdisclosure. The numerical values, shapes, materials, constituentelements, arrangement of the constituent elements, the ways in which theconstituent elements are connected to each other and so forth given inthe following embodiments are merely examples and are not intended tolimit the present disclosure. Constituent elements not described in theindependent claims among constituent elements in the followingembodiments are described as arbitrary constituent elements. Inaddition, the sizes or size ratios between the constituent elementsillustrated in the drawings are not necessarily strictly accurate.

Embodiment 1

[1.1 Circuit Configuration of Multiplexer]

FIG. 1A is a circuit configuration diagram of a multiplexer 1 accordingto embodiment 1. As illustrated in the figure, the multiplexer 1includes a low-frequency filter 11L, a high-frequency filter 12H, acapacitor C_(B1), an inductor L_(P1), a common terminal 110, andinput/output terminals 120 and 130. The multiplexer 1 is a compositeelastic wave filter device that includes the low-frequency filter 11Land the high-frequency filter 12H, which are connected to the commonterminal 110.

For example, the common terminal 110 can be connected to an antennaelement, and the input/output terminals 120 and 130 can be connected toa high-frequency signal processing circuit via an amplification circuit.

The low-frequency filter 11L is a first filter that is arranged betweenthe common terminal 110 and the input/output terminal 120 (firstinput/output terminal), and has a first pass band (center frequency f0_(11L)). The low-frequency filter 11L is configured as a ladder bandpass filter, and includes series arm resonators 101, 102, 103, 104, and105, which are connected in a series arm connected between the commonterminal 110 and the input/output terminal 120, and parallel armresonators 151, 152, 153, and 154, which are connected in parallel armsconnected between the series arm and ground terminals. The series armresonators 101 to 105 and the parallel arm resonators 151 to 154 aresurface acoustic wave (SAW) resonators. The SAW resonators of thelow-frequency filter 11L utilize leaky waves using a lithium tantalate(LiTaO₃) substrate or Love waves using a lithium niobate (LiNbO₃)substrate.

In this embodiment, an example is illustrated in which the low-frequencyfilter 11L is used as a long term evolution (LTE) standard Band 11reception filter (reception pass band: 1475.9-1495.9 MHz).

The high-frequency filter 12H is a second filter that is arrangedbetween the common terminal 110 and the input/output terminal 130(second input/output terminal) and has a second pass band (centerfrequency f0 _(12H) (>f0 _(11L))) located at a higher frequency than thefirst pass band. The high-frequency filter 12H is configured as a ladderband pass filter, and includes series arm resonators 201, 202, 203, 204,and 205, which are connected in a series arm connected between thecommon terminal 110 and the input/output terminal 130, and parallel armresonators 251, 252, 253, and 254, which are connected in parallel armsconnected between the series arm and ground terminals. In addition, theresonator that is connected closest to the common terminal 110 among theseries arm resonators 201 to 205 and the parallel arm resonators 251 to254 is the series arm resonator 201.

In this embodiment, the series arm resonators 201 to 205 and theparallel arm resonators 251 to 254 are all SAW resonators, but theseresonators may instead be elastic wave resonators that utilize boundaryacoustic waves or bulk acoustic waves (BAW). In addition, thehigh-frequency filter 12H does not need to have a ladder structure, andadditionally may have a configuration that does not include elastic waveresonators such as an LC resonance circuit.

In this embodiment, an example is illustrated in which thehigh-frequency filter 12H is used as an LTE standard Band 1 receptionfilter (reception pass band: 2110-2170 MHz)

The capacitor C_(B1) is serially arranged on a path (series arm) thatconnects the common terminal 110 and the series arm resonator 101 toeach other. Here, the Q value of the capacitor C_(B1) in the second passband is higher than a capacitance Q value, in the second pass band, of acapacitance component of the SAW resonators of the low-frequency filter11L. In other words, the capacitor C_(B1) has a function of compensatingfor bulk wave radiation loss of the SAW resonators of the low-frequencyfilter 11L in the second pass band. The capacitor C_(B1) is a principlefeature of the multiplexer 1 according to embodiment 1, and thereforethe capacitor C_(B1) will be described in detail using FIG. 2 and thefigures thereafter.

The inductor L_(P1) is connected between the common terminal 110 and aground terminal. In this way, impedance matching can be secured betweenthe antenna element and each filter.

FIG. 1B is a circuit configuration diagram of a multiplexer 1A accordingto modification 1 of embodiment 1. The multiplexer 1A according to thismodification differs from the multiplexer 1 according to embodiment 1only in terms of the circuit configuration of the low-frequency filter.Hereafter, the description of the parts of the configuration of themultiplexer 1A according to this modification that are the same as inthe configuration of the multiplexer 1 according to embodiment 1 will beomitted and the description will focus on the parts of the configurationthat are different.

As illustrated in FIG. 1B, the multiplexer 1A includes a low-frequencyfilter 13L, a high-frequency filter 12H, a capacitor C_(B2), an inductorL_(P2), a common terminal 110, and input/output terminals 120 and 130.The multiplexer 1A is a composite elastic wave filter device thatincludes the low-frequency filter 13L and the high-frequency filter 12H,which are connected to the common terminal 110.

The low-frequency filter 13L is a first filter that is arranged betweenthe common terminal 110 and the input/output terminal 120 (firstinput/output terminal) and has a first pass band (center frequency f0_(13L)). The low-frequency filter 13L is configured as a ladder bandpass filter, and includes series arm resonators 111, 112, 113, and 114,which are connected in a series arm connected between the commonterminal 110 and the input/output terminal 120, and parallel armresonators 161, 162, 163, and 164, which are connected in parallel armsconnected between the series arm and ground terminals. The series armresonators 111 to 114 and the parallel arm resonators 161 to 164 are SAWresonators. The SAW resonators of the low-frequency filter 13L utilizeleaky waves using a LiTaO₃ substrate or Love waves using a LiNbO₃substrate. The low-frequency filter 13L differs from the low-frequencyfilter 11L according to embodiment 1 in that the parallel arm resonator161 is arranged closest to the common terminal 110 and in that there arefour series arm resonators.

The capacitor C_(B2) is serially arranged on a path (series arm) thatconnects the common terminal 110 and the parallel arm resonator 161 toeach other. Here, the Q value of the capacitor C_(B2) in the second passband is higher than the capacitance Q value, in the second pass band, ofa capacitance component of the SAW resonators of the low-frequencyfilter 13L. In other words, the capacitor C_(B2) has a function ofcompensating for bulk wave radiation loss of the SAW resonators of thelow-frequency filter 13L in the second pass band.

The inductor L_(P2) is connected between the common terminal 110 and aground terminal. In this way, impedance matching can be secured betweenthe antenna element and each filter.

In other words, in a multiplexer according to the present disclosure,the resonator of the low-frequency filter that is arranged in the stagesubsequent to the capacitors C_(B1) and C_(B2) and is connected closestto the common terminal may be either a series arm resonator or aparallel arm resonator.

Next, the configuration of a multiplexer according to a comparativeexample will be described and problematic points of the multiplexeraccording to the comparative example will be described.

1.2 Multiplexer According to Comparative Example

FIG. 2 is a circuit configuration diagram of a multiplexer 600 accordingto a comparative example. As illustrated in the figure, the multiplexer600 according to the comparative example includes a low-frequency filter611L, a high-frequency filter 612H, an inductor L_(P3), a commonterminal 110, and input/output terminals 120 and 130. The multiplexer600 is a composite elastic wave filter device that includes thelow-frequency filter 611L and the high-frequency filter 612H, which areconnected to the common terminal 110. The circuit configuration of themultiplexer 600 according to the comparative example differs from thatof the multiplexer 1 according to embodiment 1 in that the capacitorC_(B1) is not arranged.

The low-frequency filter 611L is a filter that is arranged between thecommon terminal 110 and the input/output terminal 120 and has a firstpass band (center frequency f0 _(611L)). The resonator configuration ofthe low-frequency filter 611L is the same as that of the low-frequencyfilter 11L.

The high-frequency filter 612H is a filter that is arranged between thecommon terminal 110 and the input/output terminal 130 and has a secondpass band (center frequency f0 _(612H) (>f0 _(611L))) located at ahigher frequency than the first pass band. The resonator configurationof the high-frequency filter 12H is the same as that of thehigh-frequency filter 12H.

Loss due to bulk waves is generated on the high-frequency side of ananti-resonant point in a SAW resonator that utilizes leaky waves using aLiTaO₃ substrate or Love waves using a LiNbO₃ substrate.

FIG. 3A is a graph that illustrates a wide bandpass characteristic ofthe low-frequency filter 611L. In addition, FIG. 3B is a graph thatillustrates a wide reflection characteristic of the low-frequency filter611L. Furthermore, FIG. 3C is a polar chart that illustrates areflection characteristic of the low-frequency filter 611L. In thelow-frequency filter 611L formed of SAW resonators, bulk wave radiationis generated in an attenuation band on the high-frequency side of thecenter frequency f0 _(611L), and as illustrated in FIG. 3B, return lossin the case where the low-frequency filter 611L is viewed from thecommon terminal 110 side is increased in the attenuation band(frequencies greater than or equal to mark 5 in FIG. 3B). As illustratedin FIG. 3A, the loss due to the bulk waves substantially does not affectthe attenuation of the low-frequency filter 611L. However, asillustrated in FIG. 3C, the reflection coefficient (|Γ|) in theattenuation band (frequencies greater than or equal to mark 5 in FIG.3C) is reduced (|Γ|=0.84).

The low-frequency filter 611L formed of SAW resonators has a capacitivecharacteristic in the attenuation band and functions as a capacitor, andtherefore the low-frequency filter 611L functions as a low Q valuecapacitor in the frequency band where the bulk waves are generated(frequencies greater than or equal to mark 5 in FIG. 3B). Therefore, inthe case of the multiplexer 600 having a configuration in which thelow-frequency filter 611L and the high-frequency filter 612H areconnected to the common terminal 110, the filter characteristic of thehigh-frequency filter 612H, which has a pass band located at frequencieswhere the bulk wave loss of the low-frequency filter 611L is generated,is affected by the bulk wave loss of the low-frequency filter 611L,which has a lower pass band. The right-hand side of FIG. 2 illustratesan equivalent circuit diagram of the high-frequency filter 612H at thistime. In the equivalent circuit, there is a low-Q-value equivalentcapacitance component C_(LOW-Q), which arises from the bulk waveradiation of the low-frequency filter 611L, between the common terminal110 and a series arm resonator 201 of the high-frequency filter 612H.Consequently, the insertion loss inside the pass band of thehigh-frequency filter 612H is undesirably increased. In other words, amultiplexer having a configuration in which a plurality of filters aregrouped together at a common terminal has a problem in that loss insidethe pass band of the high-frequency filter is increased by bulk waveloss of the low-frequency filter.

1.3 Comparison of Characteristics of Multiplexers According toEmbodiment and Comparative Example

In order to solve the problem of the multiplexer 600 according to thecomparative example, in the multiplexer 1 according to this embodiment,as described above, the capacitor C_(B1) is serially arranged on a path(series arm) that connects the common terminal 110 and the series armresonator 101 to each other. The capacitor C_(B1) has a function ofcompensating for the bulk wave radiation loss of the SAW resonators ofthe low-frequency filter 11L in the second pass band. More specifically,the Q value of the capacitor C_(B1) in the second pass band is higherthan a capacitance Q value, in the second pass band, of a capacitancecomponent of the SAW resonators of the low-frequency filter 11L.

FIG. 4A is a graph illustrating a wide bandpass characteristic of thehigh-frequency filter 12H according to embodiment 1. FIG. 4B is a graphillustrating a wide bandpass characteristic of the high-frequency filter612H according to the comparative example.

As illustrated in FIG. 4B, in the case of the multiplexer 600 accordingto the comparative example, the maximum insertion loss inside the passband (2110-2170 MHz) is 2.638 dB (2170 MHz) in the bandpasscharacteristic of the high-frequency filter 612H between the commonterminal 110 and the input/output terminal 130 due to the bulk waveradiation of the SAW resonators constituting the low-frequency filter611L.

In contrast, as illustrated in FIG. 4A, in the case of the multiplexer 1according to embodiment 1, the maximum insertion loss inside the passband (2110-2170 MHz) is 2.119 dB (2170 MHz), which is an improvement ofaround 0.5 dB, in the bandpass characteristic of the high-frequencyfilter 12H between the common terminal 110 and the input/output terminal130 due to the addition of the capacitor C_(B1) to deal with the bulkwave radiation of the SAW resonators constituting the low-frequencyfilter 11L.

That is, according to the multiplexer 1 of embodiment 1, the propagationloss of a high-frequency signal can be reduced without arranging ademultiplexing/multiplexing device, a phase adjusting circuit, or thelike in a stage prior to the low-frequency filter 11L and thehigh-frequency filter 12H, and the reductions in size and cost can berealized.

Next, for the case where the capacitance arising from a low-frequencyfilter is equivalently added in a stage prior to a high-frequency filteras in the equivalent circuit illustrated on the right-hand side of FIG.2, the relationship between the Q value of the equivalent capacitancecomponent, the reflection coefficient (|Γ|) of the low-frequency filterin the pass band of the high-frequency filter, and the insertion loss inthe pass band of the high-frequency filter will be described using FIGS.5A to 5D.

FIG. 5A is a diagram illustrating the relationship between the Q valueof the equivalent capacitance component in the stage prior to thehigh-frequency filter, the reflection coefficient of the low-frequencyfilter, and the insertion loss of the high-frequency filter. Asillustrated in the figure, it is clear that the Q value of theequivalent capacitance component in the stage prior to thehigh-frequency filter becomes smaller and the insertion loss of thehigh-frequency filter is degraded as the reflection coefficient (|Γ|) ofthe low-frequency filter becomes smaller.

FIG. 5B is a graph illustrating the relationship between the Q value ofthe equivalent capacitance component in the stage prior to thehigh-frequency filter and the insertion loss of the high-frequencyfilter. In addition, FIG. 5C is a graph illustrating the relationshipbetween the reflection coefficient of the low-frequency filter and theinsertion loss of the high-frequency filter. Furthermore, FIG. 5D is agraph illustrating the relationship between the Q value of theequivalent capacitance component in the stage prior to thehigh-frequency filter and the reflection coefficient of thelow-frequency filter. It is clear from FIGS. 5B to 5D that in the caseof the multiplexer 600 according to the comparative example, the Q valueof the equivalent capacitance component in the stage prior to thehigh-frequency filter 612H is 5, and in the case of the multiplexer 1according to embodiment 1, the Q value of the equivalent capacitancecomponent in the stage prior to the high-frequency filter 12H is 10. Inaddition, it is clear that in the case of the multiplexer 600 accordingto the comparative example, the reflection coefficient (|Γ|) of thelow-frequency filter 611L is 0.84, and in the case of the multiplexer 1according to embodiment 1, the reflection coefficient (|Γ|) of thelow-frequency filter 11L is at least 0.9.

In other words, in the case of the multiplexer 600 according to thecomparative example, the reflection coefficient (|Γ|) of thelow-frequency filter 611L in the pass band of the high-frequency filter612H is 0.84. In contrast, in the case of the multiplexer 1 according toembodiment 1, the capacitor C_(B1) is serially arranged in the stageprior to the low-frequency filter 11L, and as a result the reflectioncoefficient (|Γ|) of the low-frequency filter 11L in the pass band ofthe high-frequency filter 12H is increased to be at least 0.9 andfurthermore the Q value of the equivalent capacitance component in thestage prior to the high-frequency filter 12H is increased from 5 to 10.As a result, the insertion loss of the high-frequency filter 12Haccording to embodiment 1 is improved from 2.638 dB to 2.119 dB. Inaddition, in this embodiment, in order to increase the Q value of theequivalent capacitance component of the high-frequency filter 12H from 5to 10, the Q value of the capacitor C_(B1) in the pass band of thehigh-frequency filter 12H was set to be around 30 to 40.

[1.4 Configuration of Capacitor]

Next, the configurations of the capacitors C_(B1) and C_(B2) accordingto embodiment 1 will be described.

FIG. 6A is a plan view illustrating a first example of the electrodelayout of the low-frequency filter 11L and the capacitor C_(B1)according to embodiment 1. As illustrated in the figure, thelow-frequency filter 11L and the capacitor C_(B1) of the multiplexer 1are formed on a substrate 100. The interdigital transducer (IDT)electrodes constituting the series arm resonators 101 to 105 and theparallel arm resonators 151 to 154 of the low-frequency filter 11L areformed on the substrate 100 such that the propagation directions of theutilized elastic waves match each other.

In addition, the capacitor C_(B1) is formed on the substrate 100 usingcomb-shaped electrodes that face each other.

The substrate 100 is a substrate having a piezoelectric property in atleast part thereof, and is for example a piezoelectric substrate, or maybe formed of a piezoelectric thin film and a support substrate.

Here, a pitch Pc of the plurality of electrode fingers constituting thecomb-shaped electrodes of the capacitor C_(B1) is smaller than a pitchλs of the plurality of electrode fingers constituting the IDT electrodesof the series arm resonators 101 to 105 and a pitch λp of the pluralityof electrode fingers constituting the IDT electrodes of the parallel armresonators 151 to 154.

In the multiplexer 1 according to this embodiment, the pitch λs of theelectrode fingers of the series arm resonators 101 to 105 of thelow-frequency filter 11L is, for example, 2.350 to 2.370 μm, and thepitch λp of the electrode fingers of the parallel arm resonators 151 to154 of the low-frequency filter 11L is, for example, 2.410 to 2.430 μm.In contrast, the pitch Pc of the electrode fingers of the capacitorC_(B1) is, for example, 1.8 μm.

Similarly to a SAW resonator, the Q value of the capacitor C_(B1) on thesubstrate 100 is also affected by bulk waves. With the above-describedconfiguration, the frequency of the bulk wave radiation generated by thecapacitor C_(B1) is shifted so as to be further toward thehigh-frequency side than the frequency of the bulk wave radiationgenerated by the low-frequency filter 11L, and therefore the Q value ofthe capacitor C_(B1) in the second pass band can be set so as to behigher than the capacitance Q value of the SAW resonators of thelow-frequency filter 11L in the second pass band. Therefore, thecapacitance Q value in the second pass band can be increased on thecommon terminal 110 side of the low-frequency filter 11L. Thus, thereflection coefficient (|Γ|) of the low-frequency filter 11L in thesecond pass band can be increased (return loss can be reduced) and theinsertion loss in the second pass band of the high-frequency filter 12Hcan be improved.

It is preferable that the direction in which the plurality of electrodefingers constituting the comb-shaped electrodes of the capacitor C_(B1)formed on the substrate 100 extend be perpendicular to (intersect) thedirection in which the plurality of electrode fingers constituting theresonators of the low-frequency filter 11L extend. As a result, thecapacitor C_(B1) suppresses the interference with elastic waves of thelow-frequency filter 11L and can function only as a capacitance element.

In addition, in the case where the high-frequency filter 12H is formedof SAW resonators, it is further preferable that the pitch of theplurality of electrode fingers constituting the comb-shaped electrodesof the capacitor C_(B1) be smaller than the pitch of the electrodefingers of the SAW resonators forming the high-frequency filter 12H.

In the multiplexer 1 according to this embodiment, the pitch λs of theelectrode fingers of the series arm resonators 201 to 205 of thehigh-frequency filter 12H is, for example, 1.980 to 2.000 μm, and thepitch λp of the electrode fingers of the parallel arm resonators 251 to254 of the high-frequency filter 12H is, for example, 2.070 to 2.090 μm.Similarly to a resonator, the Q value of a capacitor on a piezoelectricsubstrate may also be degraded by bulk waves, and therefore it ispreferable to design the capacitor so as to have a narrower pitch thanthe high-frequency filter. With this configuration, the frequency of thebulk wave radiation generated by the capacitor C_(B1) is shifted so asto be further toward the high-frequency side than the frequency of thebulk wave radiation generated by the high-frequency filter 12H, andtherefore the Q value of the capacitor C_(B1) in the second pass bandcan be set so as to be higher than the capacitance Q value of the SAWresonators of the high-frequency filter 12H in the second pass band.Therefore, the capacitance Q value in the second pass band can befurther increased on the common terminal 110 side of the low-frequencyfilter 11L. Thus, the reflection coefficient (|Γ|) of the low-frequencyfilter 11L in the second pass band can be increased (return loss can bereduced) and the insertion loss in the second pass band of thehigh-frequency filter 12H can be improved.

In addition, rather than being formed on the substrate 100, thecapacitor C_(B1) may be formed in a plurality of layers inside amultilayer substrate on which the low-frequency filter 11L and thehigh-frequency filter 12H are mounted and in which a wiring line thatconnects the low-frequency filter 11L and the high-frequency filter 12Hto each other is formed.

FIG. 6B is an example of an external perspective view of the multiplexeraccording to embodiment 1. FIG. 6C is a sectional view illustrating asecond example of the electrode layout of the low-frequency filter andthe capacitor according to embodiment 1. Specifically, FIG. 6C is asectional view taken along line VI-VI in FIG. 6B.

As illustrated in the figure, in the multiplexer 1 according toembodiment 1, the low-frequency filter 11L, the high-frequency filter12H, and the inductor L_(P1) are mounted on a mounting substrate 200.

The mounting substrate 200 has the capacitor C_(B1) and a wiring linethat connects the low-frequency filter and the high-frequency filter toeach other built thereinto and is, for example, a low-temperatureco-fired ceramic (LTCC) substrate.

As illustrated in FIG. 6C, the capacitor C_(B1) is formed across aplurality of layers in the mounting substrate 200, which is a ceramicmultilayer substrate. In addition, the mounting substrate 200 may beformed across a plurality of layers in a multilayer substrate composedof a high-temperature co-fired ceramic (HTCC) substrate or a PCB.Furthermore, a multilayer ceramic capacitor (MLCC) may be mounted on amultilayer substrate.

With this configuration, in contrast to the case where the IDTelectrodes of the SAW resonators of the low-frequency filter 11L areformed on one surface of the substrate 100, the capacitor C_(B1) isformed of a plurality of planar electrodes that face each other disposedwith a dielectric layer therebetween, and therefore the Q value of thecapacitor in the second pass band can be designed so as to be higherthan the capacitance Q values of the SAW resonators of the low-frequencyfilter 11L in the second pass band. Therefore, the capacitance Q valuein the second pass band can be increased on the common terminal 110 sideof the low-frequency filter 11L. Thus, the reflection coefficient (|Γ|)of the low-frequency filter 11L in the second pass band can be increased(return loss can be reduced) and the insertion loss in the second passband of the high-frequency filter 12H can be improved.

[1.5 Modifications of Low-Frequency Filter and High-Frequency Filter]

FIG. 7A is a circuit configuration diagram of a multiplexer 1B accordingto modification 2 of embodiment 1. As illustrated in the figure, alow-frequency filter and a high-frequency filter of a multiplexeraccording to the present disclosure may have a configuration thatincludes a longitudinally coupled SAW resonator unit. The multiplexer 1Baccording to this modification includes a low-frequency filter 15L, ahigh-frequency filter 16H, a capacitor C_(B3), an inductor L_(P4), acommon terminal 110, and input/output terminals 120 and 130. Themultiplexer 1B is a composite elastic wave filter device that includesthe low-frequency filter 15L and the high-frequency filter 16H, whichare connected to the common terminal 110.

The low-frequency filter 15L is a first filter that is arranged betweenthe common terminal 110 and the input/output terminal 120 (firstinput/output terminal) and has a first pass band (center frequency f0_(15L)). The low-frequency filter 15L is configured as a band passfilter, and includes series arm resonators 121 and 122, which areconnected in a series arm connected between the common terminal 110 andthe input/output terminal 120, a parallel arm resonator 171 that isconnected in a parallel arm connected between the series arm and aground terminal, and a longitudinally coupled SAW resonator unit 125that is connected to the series arm resonators 121 and 122. The seriesarm resonators 121 and 122, the parallel arm resonator 171, and the fiveresonators constituting the longitudinally coupled SAW resonator unit125 are SAW resonators. The SAW resonators of the low-frequency filter15L utilize leaky waves using a LiTaO₃ substrate or Love waves using aLiNbO₃ substrate.

The high-frequency filter 16H is a second filter that is arrangedbetween the common terminal 110 and the input/output terminal 130(second input/output terminal) and has a second pass band (centerfrequency f0 _(16H) (>f0 _(15L))) located at a higher frequency than thefirst pass band. The high-frequency filter 16H is configured as a bandpass filter, and includes series arm resonators 211, 212, and 213, whichare connected in a series arm connected between the common terminal 110and the input/output terminal 130, a parallel arm resonator 261, whichis connected in a parallel arm connected between the series arm and aground terminal, and a longitudinally coupled SAW resonator unit 215,which is connected to the series arm resonators 212 and 213. Inaddition, the resonator that is connected closest to the common terminal110 among the resonators is the series arm resonator 211. The series armresonators 211, 212, and 213, the parallel arm resonator 261, and thefive resonators constituting the longitudinally coupled SAW resonatorunit 215 are SAW resonators. In addition, the resonators may be elasticwave resonators that utilize boundary acoustic waves or BAW. Inaddition, the high-frequency filter 16H may have a configuration thatdoes not include elastic wave resonators such as an LC resonancecircuit.

The capacitor C_(B3) is serially arranged on a path (series arm) thatconnects the common terminal 110 and the parallel arm resonator 171 toeach other. Here, the Q value of the capacitor C_(B3) in the second passband is higher than capacitance Q value, in the second pass band, of acapacitance component of the SAW resonators of the low-frequency filter15L. In other words, the capacitor C_(B3) has a function of compensatingfor bulk wave radiation loss of the SAW resonators of the low-frequencyfilter 15L in the second pass band.

The inductor L_(P4) is connected between the common terminal 110 and aground terminal. In this way, impedance matching can be secured betweenthe antenna element and each filter.

With this configuration, the reflection coefficient (|Γ|) of thelow-frequency filter 15L in the second pass band can be increased(return loss can be reduced), and therefore the insertion loss in thesecond pass band of the high-frequency filter 16H, which is connected tothe common terminal 110 along with the low-frequency filter 15L, can beimproved. Therefore, the propagation loss of a high-frequency signal canbe reduced without arranging a demultiplexing/multiplexing device, aphase adjusting circuit, or the like in a stage prior to the filters,and the reductions in size and cost can be realized.

FIG. 7B is a circuit configuration diagram of a multiplexer 1C accordingto modification 3 of embodiment 1. As illustrated in the figure, alow-frequency filter 17L may be a ladder SAW filter and a high-frequencyfilter 18H may be a SAW filter that includes a longitudinally coupledSAW resonator unit 225.

The multiplexer 1C according to this modification includes thelow-frequency filter 17L, the high-frequency filter 18H, a capacitorC_(B4), an inductor L_(P5), a common terminal 110, and input/outputterminals 120 and 130.

The low-frequency filter 17L is a first filter that has a first passband (center frequency f0 _(17L)). The low-frequency filter 17L isconfigured as a ladder band pass filter, and includes series armresonators 131, 132, 133, and 134, which are connected in a series armconnected between the common terminal 110 and the input/output terminal120, and parallel arm resonators 181, 182, 183, and 184, which areconnected in parallel arms connected between the series arm and groundterminals.

The high-frequency filter 18H is a second filter having a second passband (center frequency f0 _(18H) (>f0 _(17L))) located at a higherfrequency than the first pass band and has the same configuration as thehigh-frequency filter 16H according to modification 2. Thehigh-frequency filter 18H is configured as a band pass filter, andincludes series arm resonators 221, 222, and 223, which are connected ina series arm connected between the common terminal 110 and theinput/output terminal 130, a parallel arm resonator 271 that isconnected in a parallel arm connected between the series arm and aground terminal, and a longitudinally coupled SAW resonator unit 225that is connected to the series arm resonators 222 and 223. In addition,the resonator that is connected closest to the common terminal 110 amongthe resonators is the series arm resonator 221.

With this configuration, the reflection coefficient (|Γ|) of thelow-frequency filter 17L in the second pass band can be increased(return loss can be reduced), and therefore the insertion loss in thesecond pass band of the high-frequency filter 18H, which is connected tothe common terminal 110 along with the low-frequency filter 17L, can beimproved. Therefore, the propagation loss of a high-frequency signal canbe reduced without arranging a demultiplexing/multiplexing device, aphase adjusting circuit, or the like in a stage prior to the filters,and the reductions in size and cost can be realized.

Embodiment 2

In embodiment 1, a two-branch demultiplexing/multiplexing multiplexerhaving a configuration in which a low-frequency filter and ahigh-frequency filter are connected to a common terminal is exemplified,whereas a three-branch demultiplexing/multiplexing multiplexer having aconfiguration in which three filters having different pass bands areconnected to a common terminal will be described in this embodiment.

[2.1 Circuit Configuration of Multiplexer]

FIG. 8A is a circuit configuration diagram of a multiplexer 2 accordingto embodiment 2. As illustrated in the figure, the multiplexer 2includes a low-band filter 21L, a middle-band filter 22M, a high-bandfilter 23H, capacitors C_(BL) and C_(BM), a common terminal 110, andinput/output terminals 120, 130, and 140. The multiplexer 2 is acomposite elastic wave filter device that includes the low-band filter21L, the middle-band filter 22M, and the high-band filter 23H, which areconnected to the common terminal 110.

For example, the common terminal 110 can be connected to an antennaelement and the input/output terminals 120, 130, and 140 can beconnected to a high-frequency signal processing circuit via anamplification circuit.

The low-band filter 21L is a first filter that is arranged between thecommon terminal 110 and the input/output terminal 120 (firstinput/output terminal) and has a first pass band (center frequency f0_(21L)). The low-band filter 21L is formed of at least one SAWresonator.

The middle-band filter 22M is a third filter that is arranged betweenthe common terminal 110 and the input/output terminal 130 (firstinput/output terminal) and has a third pass band (center frequency f0_(22M)) located at a higher frequency than the first pass band. Themiddle-band filter 22M is formed of at least one SAW resonator.

The high-band filter 23H is a second filter that is arranged between thecommon terminal 110 and the input/output terminal 140 (secondinput/output terminal) and has a second pass band (center frequency f0_(23H) (>f0 _(22M))) located at higher frequencies than the third passband. The high-band filter 23H is formed of at least one SAW resonator.In addition, in this embodiment, the resonators constituting thehigh-band filter 23H are all SAW resonators, but the resonators mayinstead be elastic wave resonators that utilize boundary acoustic wavesor BAW. Furthermore, the high-band filter 23H may have a configurationthat does not include elastic wave resonators such as an LC resonancecircuit.

The capacitor C_(BL) is serially arranged on a path (series arm) thatconnects the common terminal 110 and the low-band filter 21L to eachother. Here, the Q valued of the capacitor C_(BL) in the second passband and the third pass band are higher than the capacitance Q values ofthe capacitance component of the SAW resonators of the low-band filter21L in the second pass band and the third pass band. In other words, thecapacitor C_(BL) has a function of compensating for bulk wave radiationloss of the SAW resonators of the low-band filter 21L in the second passband and the third pass band.

The capacitor C_(BM) is serially arranged on a path (series arm) thatconnects the common terminal 110 and the middle-band filter 22M to eachother. Here, the Q value of the capacitor C_(BM) in the second pass bandis higher than the capacitance Q value of the capacitance component ofthe SAW resonators of the middle-band filter 22M in the second passband. In other words, the capacitor C_(BM) has a function ofcompensating for the bulk wave radiation loss of the SAW resonators ofthe middle-band filter 22M in the second pass band.

FIG. 8B is a circuit configuration diagram of a multiplexer 2A accordingto embodiment 2. The multiplexer 2A illustrated in the figure representsa specific circuit configuration of the multiplexer 2. As illustrated inthe figure, the multiplexer 2A includes the low-band filter 21L, themiddle-band filter 22M, the high-band filter 23H, capacitors C_(B5) andC_(B6), an inductor L_(P6), the common terminal 110, and theinput/output terminals 120, 130, and 140.

The low-band filter 21L has the same configuration as the low-frequencyfilter 11L according to embodiment 1. The SAW resonators of the low-bandfilter 21L utilize leaky waves using a LiTaO₃ substrate or Love wavesusing a LiNbO₃ substrate.

The middle-band filter 22M is configured as a ladder band pass filter,has the same resonator configuration as the low-band filter 21L, andincludes series arm resonators 201 to 205, which are connected in aseries arm connected between the common terminal 110 and theinput/output terminal 130, and parallel arm resonators 251 to 254, whichare connected in parallel arms connected between the series arm and theground terminals. The SAW resonators of the middle-band filter 22Mutilize leaky waves using a LiTaO₃ substrate or Love waves using aLiNbO₃ substrate.

The high-band filter 23H is configured as a ladder band pass filter, hasthe same configuration as the high-frequency filter 12H according toembodiment 1, and includes series arm resonators 301 to 305, which areconnected in a series arm connected between the common terminal 110 andthe input/output terminal 130, and parallel arm resonators 351 to 354,which are connected in parallel arms connected between the series armand ground terminals. In addition, the resonator that is connectedclosest to the common terminal 110 among the series arm resonators 301to 305 and the parallel arm resonators 351 to 354 is the series armresonator 301.

The capacitor C_(B5) is the capacitor C_(BL) in the multiplexer 2 andthe capacitor C_(B6) is the capacitor C_(BM) in the multiplexer 2.

The inductor L_(P6) is connected between the common terminal 110 and aground terminal. In this way, impedance matching can be secured betweenthe antenna element and each filter.

Due to the capacitor C_(B5) being arranged, the reflection coefficient(|Γ|) of the low-band filter 21L in the second pass band and the thirdpass band can be increased (return loss can be reduced), and thereforethe insertion loss in the second pass band of the high-band filter 23Hand in the third pass band of the middle-band filter 22M, these filtersbeing connected to the common terminal 110 along with the low-bandfilter 21L, can be improved.

In addition, due to the capacitor C_(B6) being arranged, the reflectioncoefficient (|Γ|) of the middle-band filter 22M in the second pass bandcan be increased (return loss can be reduced), and therefore theinsertion loss in the second pass band of the high-band filter 23H,which is connected to the common terminal 110 along with the middle-bandfilter 22M, can be improved.

With the above-described configuration, the propagation loss of ahigh-frequency signal can be reduced without arranging a three-branchdemultiplexing/multiplexing device, a phase adjusting circuit, and so onin a stage prior to the filters, and the reductions in size and cost canbe realized.

FIG. 8C is a circuit configuration diagram of a multiplexer 2B accordingto modification 1 of embodiment 2. The multiplexer 2B illustrated in thefigure differs from the multiplexer 2A according to embodiment 2 onlywith respect to the circuit configurations of the low-band filter 21Land the middle-band filter 22M. Hereafter, only the parts of theconfiguration of the multiplexer 2B according to this modification thatdiffer from those of the multiplexer 2A according to embodiment 2 willbe described.

A low-band filter 21L according to this modification has the sameconfiguration as the low-frequency filter 13L according to modification1 of embodiment 1.

The middle-band filter 22M according to this modification has the sameresonator configuration as the low-band filter 21L according to thismodification.

A capacitor C_(B7) is the capacitor C_(BL) in the multiplexer 2 and acapacitor C_(B8) is the capacitor C_(BM) in the multiplexer 2.

In other words, in a multiplexer according to the present disclosure,the resonators of the low-band filter 21L and the middle-band filter 22Mthat are arranged in the stage after the capacitors C_(BL) and C_(BM)and are connected closest to the common terminal may each be a seriesarm resonator or a parallel arm resonator.

FIG. 8D is a circuit configuration diagram of a multiplexer 2C accordingto modification 2 of embodiment 2. The multiplexer 2C illustrated in thefigure differs from the multiplexer 2A according to embodiment 2 onlywith respect to the configurations of the low-band filter 21L and themiddle-band filter 22M. Hereafter, only the parts of the configurationof the multiplexer 2C according to this modification that differ fromthose of the multiplexer 2A according to embodiment 2 will be described.

A low-band filter 21L according to this modification has the sameconfiguration as the low-band filter 21L according to modification 1 ofembodiment 2. Here, the SAW resonators of the low-band filter 21Laccording to this modification utilizes Rayleigh waves that propagatealong a LiTaO₃ piezoelectric substrate. In addition, the SAW resonatorsof the low-band filter 21L according to this modification may insteadutilizes Love wave that propagate along a LiNbO₃ piezoelectricsubstrate.

A capacitor C_(B9) is the capacitor C_(BL) in the multiplexer 2.

Due to the capacitor C_(B9) being arranged, the reflection coefficient(|Γ|) of the low-band filter 21L in the second pass band can beincreased (return loss can be reduced), and therefore the insertion lossin the second pass band of the high-band filter 23H, which is connectedto the common terminal 110 along with the low-band filter 21L, can beimproved.

The middle-band filter 22M is configured as a ladder band pass filter,has the same resonator configuration as the middle-band filter 22Maccording to embodiment 2, and includes series arm resonators 241 to245, which are connected in a series arm connected between the commonterminal 110 and the input/output terminal 130, and parallel armresonators 281 to 284, which are connected in parallel arms connectedbetween the series arm and ground terminals. Here, the SAW resonators ofthe middle-band filter 22M according to this modification utilizeRayleigh waves that propagate along a LiNbO₃ piezoelectric substrate,and a capacitor is not serially arranged on a path connected between thecommon terminal 110 and the middle-band filter 22M.

In a SAW resonator utilizing Rayleigh waves using a LiNbO₃ piezoelectricsubstrate, the frequency at which bulk wave radiation is generated onthe high-frequency side of the anti-resonant point lies in a frequencyband located at a frequency at least twice the frequency of theanti-resonant point and is sufficiently higher than the pass band of afilter used in a cellular phone multiplexer, and there is substantiallyno effect from the bulk wave radiation on other filters. As a result,the reflection coefficient (|Γ|) of the middle-band filter 22M accordingto this modification in the second pass band is not reduced by the bulkwave radiation. Therefore, the propagation loss of a high-frequencysignal can be reduced without arranging a capacitor in the stage priorto the middle-band filter 22M according to this modification, and thereductions in size and cost can be realized.

FIG. 8E is a circuit block diagram of a multiplexer 3 according tomodification 3 of embodiment 2. As illustrated in the figure, themultiplexer 3 according to this modification includes a low-band filter31L, a middle-band filter 32M, a high-band filter 33H, capacitors C_(BL)and C_(BM), a common terminal 110, input/output terminals 120, 130, and140, and switches 31 s, 32 s, and 33 s. The multiplexer 3 is a compositeelastic wave filter device that includes the low-band filter 31L, themiddle-band filter 32M, and the high-band filter 33H, which areconnected to the common terminal 110.

The multiplexer 3 according to this modification differs from themultiplexer 2 according to embodiment 2 in that the switches 31 s, 32 s,and 33 s are respectively arranged along paths that connect the commonterminal 110, the low-band filter 31L, the middle-band filter 32M, andthe high-band filter 33H to each other. Hereafter, only the parts of theconfiguration of the multiplexer 3 according to this modification thatdiffer from those of the multiplexer 2 according to embodiment 2 will bedescribed.

As described above, the switches 31 s, 32 s, and 33 s, which switch thesignal paths between being connected and disconnected, are arrangedbetween the common terminal 110 and the filters, and the switches 31 sto 33 s are controlled in accordance with the frequency band (band) tobe used, and consequently there is no need to consider the reflectioncoefficient of a filter on a line disconnected by a switch. Therefore,the insertion loss in the pass band of each filter can be improved.

FIG. 8F is a circuit configuration diagram of a multiplexer 3A accordingto modification 3 of embodiment 2. The multiplexer 3A illustrated in thefigure exemplifies a specific circuit configuration of the multiplexer3. As illustrated in the figure, the multiplexer 3A includes thelow-band filter 31L, the middle-band filter 32M, the high-band filter33H, capacitors C_(B10) and C_(B11), an inductor L_(P9), the commonterminal 110, the input/output terminals 120, 130, and 140, and theswitches 31 s, 32 s, and 33 s.

The low-band filter 31L has the same configuration as the low-frequencyfilter 21L according to embodiment 2. The SAW resonators of the low-bandfilter 21L utilize leaky waves using a LiTaO₃ substrate or Love wavesusing a LiNbO₃ substrate.

The middle-band filter 32M is configured as a band pass filter, andincludes longitudinal coupled SAW resonance units 291 and 292 connectedin a series arm connected between the common terminal 110 and theinput/output terminal 130. The six resonators that form the longitudinalcoupled SAW resonance units 291 and 292 are SAW resonators. The SAWresonators of the middle-band filter 32M utilize leaky waves using aLiTaO₃ substrate or Love waves using a LiNbO₃ substrate.

The high-band filter 33H is configured as a ladder band pass filter, hasthe same configuration as the high-frequency filter 23H according toembodiment 2, and includes the series arm resonators 301 to 305, whichare connected in a series arm connected between the common terminal 110and the input/output terminal 130, and the parallel arm resonators 351to 354, which are connected in parallel arms connected between theseries arm and ground terminals. In addition, the resonator that isconnected closest to the common terminal 110 among the series armresonators 301 to 305 and the parallel arm resonators 351 to 354 is theseries arm resonator 301.

The capacitor C_(B10) is the capacitor C_(BL) in the multiplexer 3 andthe capacitor C_(B11) is the capacitor C_(BM) in the multiplexer 3.

The inductor L_(P9) is connected between the common terminal 110 and aground terminal. In this way, impedance matching can be secured betweenthe antenna element and each filter.

The switches 31 s, 32 s, and 33 s, which switch signal paths betweenbeing connected and disconnected, are arranged between the commonterminal 110 and the filters as in this configuration, the switches 31 sto 33 s are controlled in accordance with the frequency band (band) tobe used, and consequently there is no need to consider the reflectioncoefficient of a filter on a line disconnected by a switch. Therefore,the insertion loss in the pass band of each filter can be improved.

Embodiment 3

The multiplexers according to embodiments 1 and 2 and the modificationsthereof described thereabove can also be applied to a front end circuitand to a communication device that includes such a front end circuit.Accordingly, such a high-frequency front end circuit and such acommunication device will be described in this embodiment.

FIG. 9 is a circuit configuration diagram of a high-frequency front endcircuit 30 and a communication device 40 according to embodiment 3. Inthis figure, an antenna element 5 connected to the communication device40 is also illustrated. The communication device 40 is formed of thehigh-frequency front end circuit 30, an RF signal processing circuit(RFIC) 6, and a baseband signal processing circuit (BBIC) 7.

The high-frequency front end circuit 30 includes a multiplexer 1, aswitch 25, and a low-noise amplification circuit 26.

The multiplexer 1 is the multiplexer 1 according to embodiment 1, forexample.

The switch 25 is a switch circuit having two selection terminals thatare individually connected to the input/output terminals 120 and 130 ofthe multiplexer 1 and a common terminal that is connected to thelow-noise amplification circuit 26. The switch 25 is formed of a singlepole double throw (SPDT) switch for example, and connects the commonterminal and a signal path corresponding to a prescribed band inaccordance with a control signal from a control unit (not illustrated).The number of selection terminals connected to the common terminal isnot limited to one and may be plural. In other words, the high-frequencyfront end circuit 30 may support carrier aggregation.

The low-noise amplification circuit 26 is a reception amplificationcircuit that amplifies a high-frequency signal (in this case,high-frequency reception signal) received via the antenna element 5, themultiplexer 1, and the switch 25 and outputs the amplified signal to theRF signal processing circuit 6.

The RF signal processing circuit 6 subjects a high-frequency receptionsignal input thereto from the antenna element 5 via a reception signalpath to signal processing using down conversion and so forth, andoutputs the reception signal generated through this signal processing tothe baseband signal processing circuit 7. The RF signal processingcircuit 6 is an RFIC, for example.

A signal processed by the baseband signal processing circuit 7 is usedfor image display as an image signal or for a phone call as an audiosignal, for example.

The high-frequency front end circuit 30 may include other circuitelements between the above-described constituent elements.

The thus-configured high-frequency front end circuit 30 andcommunication device 40 include a multiplexer according to embodiment 1or 2 or a modification thereof, and as a result, the propagation loss ofa high-frequency signal can be reduced without arranging ademultiplexing/multiplexing device, a phase adjusting circuit, or thelike in a stage prior to the filters, and the reductions in size andcost can be realized.

The high-frequency front end circuit 30 may include a triplexer or aquadplexer capable of handling both transmission and reception insteadof the multiplexer 1 according to embodiment 1.

In addition, depending on the high-frequency signal processing methodused, the communication device 40 may not have to include the basebandsignal processing circuit 7.

(Other Modifications Etc.)

Multiplexers, a high-frequency front end circuit, and a communicationdevice according to embodiments of the present disclosure have beendescribed above in the form of embodiments and modifications thereof,but other embodiments realized by combining any of the constituentelements of the above-described embodiments and modifications with oneanother, modifications obtained by modifying the above-describedembodiments in various ways, as thought of by one skilled in the art,without departing from the gist of the present disclosure, and variousdevices having a high-frequency filter circuit and a front-end moduleaccording to the present disclosure built thereinto are also included inthe present disclosure.

For example, in the above-described disclosures, a two-branchdemultiplexing/multiplexing circuit in which two reception signal pathsare connected to a common terminal and a three-branchdemultiplexing/multiplexing circuit in which three reception signalpaths or transmission signal paths are connected to a common terminalare described as examples of a multiplexer, but the present disclosurecan also be applied to a circuit that includes both transmission pathsand reception paths and a demultiplexing/multiplexing circuit in whichfour or more signal paths are connected to a common terminal, forexample.

In other words, in a multiplexer in which (n) filters having centerfrequencies f1, f2, . . . , fn (n is a natural number greater than orequal to 2) are connected to a common terminal, in a first filter, whichis at least one filter other than a filter having the highest centerfrequency fn (second filter), a capacitor is serially arranged on aconnection path (series arm) connected between the common terminal andthe first filter. In this case, the Q value of the capacitor in the passband of the second filter (second pass band) is higher than acapacitance Q value in the second pass band obtained when the SAWresonators of the first filter are viewed as a capacitance. In otherwords, the capacitor has a function of compensating for bulk waveradiation loss of the SAW resonators of the first filter in the secondpass band.

As a result, the reflection coefficient (|Γ|) of the first filter in thesecond pass band can be increased (return loss can be reduced), andtherefore the insertion loss in the second pass band of the secondfilter, which is connected to the common terminal along with the firstfilter, can be improved. Therefore, the propagation loss of ahigh-frequency signal can be reduced without arranging ademultiplexing/multiplexing device, a phase adjusting circuit, or thelike in a stage prior to the filters, and the reductions in size andcost can be realized.

Furthermore, Band 1 and Band 11 of the LTE standard are used as anexample of the frequency bands (bands) used in a multiplexer in theabove-described embodiments, but the present disclosure is not limitedto this example.

In addition, in the above-described embodiments, the meaning of “two ormore filters are connected to a common terminal” includes not only aconfiguration in which two or more filters are directly connected to acommon terminal but also includes a configuration in which two or morefilters are indirectly connected to a common terminal using thefollowing kind of configuration. For example, a configuration may beadopted in which a branching circuit that enables one or more conductivepaths to be obtained such as a switch, a phase circuit or a powersplitter (divider) is arranged between the common terminal and two ormore filters.

In addition, in each filter of a multiplexer, additionally, an inductoror a capacitor may be connected between terminals such as aninput/output terminal and a ground terminal or a circuit element otherthan an inductor or a capacitor such as a resistance element may beadded.

The present disclosure can be widely used in communication devices suchas cellular phones as a multiplexer, a high-frequency front end circuit,and a communication device that can be applied to frequency standardsthat support multiple bands and multiple modes and that have low loss,are compact, and are low cost.

-   -   1, 1A, 1B, 1C, 2, 2A, 2B, 2C, 600 multiplexer    -   5 antenna element    -   6 RF signal processing circuit (RFIC)    -   7 baseband signal processing circuit (BBIC)    -   11L, 13L, 15L, 17L, 611L low-frequency filter    -   12H, 16H, 18H, 612H high-frequency filter    -   21L low-band filter    -   22M middle-band filter    -   23H high-band filter    -   25 switch    -   26 low-noise amplification circuit    -   30 high-frequency front end circuit    -   40 communication device    -   100 substrate    -   101, 102, 103, 104, 105, 111, 112, 113, 114, 121, 122, 131, 132,        133, 134, 201, 202, 203, 204, 205, 211, 212, 213, 221, 222, 223,        241, 242, 243, 244, 245, 301, 302, 303, 304, 305 series arm        resonator    -   110 common terminal    -   120, 130, 140 input/output terminal    -   125, 215, 225 longitudinally coupled SAW resonator unit    -   151, 152, 153, 154, 161, 162, 163, 164, 171, 181, 182, 183, 184,        251, 252, 253, 254, 261, 271, 281, 282, 283, 284, 351, 352, 353,        354 parallel arm resonator    -   C_(B1), C_(B2), C_(B3), C_(B4), C_(B5), C_(B6), C_(B7), C_(B8),        C_(B9), C_(BL), C_(BM) capacitor    -   L_(P1), L_(P2), L_(P3), L_(P4), L_(P5), L_(P6), L_(P7), L_(P8)        inductor

The invention claimed is:
 1. A multiplexer comprising: a commonterminal; a first input/output terminal; a second input/output terminal;a plurality of filters connected to the common terminal, the pluralityof filters including a first filter and a second filter, wherein: thefirst filter has a first pass band and is formed from at least onesurface acoustic wave resonator arranged between the common terminal andthe first input/output terminal, and the second filter has a second passband and is connected between the common terminal and the secondinput/output terminal, the second pass band having a higher frequencythan the first pass band; and a capacitor that is serially arranged on apath between the common terminal and the first filter and that isconfigured to compensate for bulk wave radiation loss of the at leastone surface acoustic wave resonator in the second pass band.
 2. Themultiplexer according to claim 1, wherein: the first filter comprises asubstrate having a piezoelectric property and on which an interdigitaltransducer (IDT) electrode constituting the at least one surfaceacoustic wave resonator is formed, the capacitor is formed on thesubstrate with comb-shaped electrodes that face each other, and a pitchof a plurality of electrode fingers constituting the comb-shapedelectrodes is less than a pitch of a plurality of electrode fingersconstituting the IDT electrode.
 3. The multiplexer according to claim 1,further comprising: a wiring line that connects the first filter and thesecond filter to each other, wherein: the capacitor is formed in aplurality of layers inside a multilayer substrate, the first filter andthe second filter are mounted on the multilayer substrate, and thewiring line is formed in the multilayer substrate.
 4. The multiplexeraccording to claim 1, wherein surface acoustic waves of the at least onesurface acoustic wave resonator are leaky waves that propagate along apiezoelectric substrate composed of lithium tantalate (LiTaO₃).
 5. Themultiplexer according to claim 1, wherein surface acoustic waves of theat least one surface acoustic wave resonator are Love waves thatpropagate along a piezoelectric substrate composed of lithium niobate(LiNbO₃).
 6. The multiplexer according to claim 1, further comprising: athird input/output terminal; and a third filter of the plurality offilters, the third filter having a third pass band and being formed fromat least one surface acoustic wave resonator arranged between the commonterminal and the third input/output terminal, wherein: the third passband has a lower frequency than the second pass band, surface acousticwaves of the at least one surface acoustic wave resonator of the thirdfilter are Rayleigh waves that propagate along a piezoelectric substratecomposed of lithium niobate (LiNbO₃), and there is no capacitor seriallyarranged on a path between the common terminal and the third filter. 7.The multiplexer according to claim 1, further comprising: a switchconfigured to switch an electrical connection of a signal path betweenthe common terminal and one of the plurality of filters.
 8. Themultiplexer according to claim 1, further comprising: an inductor thatis connected between the common terminal and a ground terminal.
 9. Ahigh-frequency front end circuit comprising: the multiplexer accordingto claim 1; and an amplification circuit that is connected to themultiplexer.
 10. A communication device comprising: a radio frequency(RF) signal processing circuit configured to process a high-frequencysignal transmitted or received by an antenna; and the high-frequencyfront end circuit according to claim 9 configured to transmit thehigh-frequency signal between the antenna and the RF signal processingcircuit.